Sift flour, cocoa and baking soda into a medium bowl to remove any lumps. Add espresso powder and salt and whisk to blend evenly.
Combine butter with both sugars and vanilla in the bowl of a stand mixer fitted with the paddle attachment and beat on medium speed for 2 minutes until slightly fluffy and lightened in colour by a shade. Stop to scrape down the sides of the bowl a few times during mixing. You can also use an electric hand mixer.
Beat in egg until well combined. Add dry ingredients and mix on low until it just begins to incorporate then fold in the chocolate chunks until they are evenly distributed. Cover the bowl and refrigerate for 1 hour.
Preheat oven to 350°F. Line 2 large baking sheets with parchment paper.
Scoop up about 2 tablespoons of dough and roll into smooth balls. Place onto prepared baking sheets spacing them 2 inches apart. Place a few extra chocolate chunks on top of each dough ball if desired. Flatten just slightly and bake for 9-11 minutes until puffed and the tops are cracked but they are still soft in the middle. Transfer to a rack and let cookies cool on trays for 3 minutes before transferring individually to the rack to finish cooling.